1. Technical Field of the Invention
The present invention relates, generally, to a driving apparatus for an oscillating catcher of a sewing machine.
More particularly, the present invention relates to a driving apparatus for an oscillating catcher of a sewing machine, which is capable of providing greater stitching security and/or a greater stitching width during the sewing of a zig-zag stitching procedure than comparable devices currently known to the art.
2. Description of the Prior Art
Various designs for driving devices for oscillating catchers are known in the prior art. The function of such driving devices is to place the catcher of the sewing machine into an oscillating rotational motion for the purpose of seizing with the point of the catcher of the loop of the upper thread, as well as for guiding the catcher about the spool capsule. The catcher is driven, in most cases, with a crank, which is put into motion by a revolving, or oscillating driving element. In many cases, the crank is driven via a toothed rack, or a toothed gear, mounted on a driving shaft of the catcher.
In Switzerland Patent No. 241,880, the toothed rack is guided tangentially relative to the toothed gear in a longitudinal guide, and driven via a connecting rod secured on a crank.
In United Kingdom Patent Specification No. 1,035,881, the teeth of the toothed rack of the apparatus disclosed therein are directly formed on an extension of a connecting rod. The connecting rod is put into an oscillating motion via a swinging lever. The two driving devices taught in this prior art reference can be employed for both horizontally and vertically oscillating catchers.
The driving devices of the types specified in the foregoing prior art have the distinct drawback that an attainable rotational speed for the catcher is relatively low at the moment that the loop is picked up by the point of the catcher. However, the rotational speed of the catcher cannot be varied at will with conventional devices known to the prior art, because the driving movements of the needle bar and of the catcher of a double-stitch sewing machine have to be synchronized. As long as the needle moves in a central position, i.e., in the zero position, and is therefore not deflected sideways for zig-zag sewing, the movements of the catcher exactly match the position of the needle.
The loop of the upper thread can be securely seized and uniformly guided. When zig-zag seams are sewn, the phase position between the eye of the needle and the point of the catcher changes at the moment that the loop is taken over by the catcher, depending upon the contemporaneous lateral position of the needle, i.e., at the moment of its lateral deflection from the center position. This is because, while the point of the catcher is moving past the needle for seizing the loop of the upper thread offered by the needle, the needle is already moving upwards. This lateral displacement of the needle during zig-zag sewing necessarily changes the spacing between the eye of the needle and the point of the catcher. If this spacing is smaller or larger than the nominal (or rated) spacing, faulty stitches may occur, because the shape and size of the loop of the thread changes with the deflection, as well, and the point of the catcher will not pick up the loop in the most unfavorable case.
The maximally achievable zig-zag width, i.e., the deflection of the needle, is influenced by, and/or upwardly limited by, the deviation of the phase position between the eye of the needle and the point of the catcher. The effects of such phase displacement are described and illustrated in the handbook by Renters, entitled Der Naehmaschinen-Fachmann, which translates to The Sewing Machine Expert, in English. The relevant portions of this reference are found in Volume 3, published in 1957.
In summation, it becomes clear that a greater zig-zag width would be attainable if the deviations from the conditions present when a straight seam is sewn did not exist.
The foregoing discussion makes clear that the stitching security and the maximally achievable stitching width during zig-zag sewing are very strongly influenced by the speed of the relative movements between the needle and the catcher. Both the translatory movement of the needle bar and the needle, and the oscillating motion of the catcher take place in non-uniform ways, conditioned by the movement pattern (acceleration/deceleration) of their crank drives. In order to improve the motional conditions within the range in which the loop of the thread is picked up by the point of the catcher, or to reduce the phase shift between the eye of the needle and the point of the catcher in said range, it would be necessary to either retard the movement of the needle or to increase the speed at which the catcher is moving.